Special uses of invisbar

The "invisbar" can be used
to force Mup into
doing something in the middle of a bar that it normally would allow
to happen only at a bar line.
Suppose, for example, you wish to place a
rehearsal letter
in the middle of a measure. This could be accomplished as follows:

// assume we are in 4/4 time, but want
// a rehearsal letter by count 3
// of the measure
// do first part of measure and use "space"
// for last part so time values will add up
// properly to a full measure
1: 4c;d;2s;
// put in invisible bar with rehearsal letter
invisbar rehearsal let
// now do the last half of the measure,
// this time with space at the beginning
1: 2s;4e;c;
bar

As another example of invisbar use, suppose you want to add a "courtesy"
key signature at the end of a repeated section to remind the player that the
beginning of the repeated section is in a different key. This can be done
by adding an empty measure whose sole purpose is to produce this key signature.